1. Field of the Invention
The present invention relates to a micro secure digital adapter, and more particularly, to a micro secure digital adapter for transmitting high speed signals with low crosstalk.
2. Description of the Prior Art
Micro secure digital (SD) is a non-volatile memory card format derived from secure digital with a smaller size and less pins. Thus, micro SD cards are widely used in electronic devices, such as mobile phones, digital cameras, GPS navigation devices, tablet computers, etc. However, some electronic devices only support SD interface, but does not support micro SD interface, such that a micro SD adapter is necessary to transform signals between a micro SD card and the electronic device.
Except the size, the main difference between interfaces of SD and micro SD is the pin counts. SD has nine pins including two ground pins, while micro SD has eight pins including only one ground pin. Therefore, the conventional micro SD adapter usually connects the two ground pins of the SD side (near the host side) via a strip or bridge. In other words, one of the two ground pins of the SD side connects to a ground pin of the micro SD card via another ground pin first, which causes unbalance between routes from the two ground pins to the ground pin of the micro SD card. In such a situation, the return loss for signals in the data pins near the ground pin with longer route to the micro SD card is increased and the signal quality thereof is further degraded.
In detail, please refer to FIG. 11, which is a schematic diagram of a micro SD adapter 70 according to the prior art. The micro SD adapter 70 has connectors CN1-CN8, data pins PIN_D0-PIN_D3, ground pins PIN_GND1-PIN_GND2, a clock pin PIN_CLK, a voltage pin PIN_VDD and a command pin PIN_CMD. The data pins PIN_D0-PIN_D3, the ground pins PIN_GND1-PIN_GND2, the clock pin PIN_CLK, the voltage pin PIN_VDD and the command pin PIN_CMD conform to SD specification for connecting to an SD interface of a host device and transmitting signals between the micro SD card and the host device via the connectors CN1-CN8. Moreover, the ground pin PIN_GND1 connects to the ground pins PIN_GND2 via a conducting strip S, so that a route R2 from the ground pin PIN_GND2 through the conducting strip S and the ground pin PIN_GND1 to a terminal T1 of the connector CN3 is longer than a route R1 from the ground pin PIN_GND1 to the terminal T1 of the connector CN3, i.e. R2≅R1+S. Therefore, the return loss for signals in the data pins PIN_D2 and PIN_D3 near the ground pin PIN_GND2 is increased and the signal quality thereof is further degraded.
Besides, the conventional micro SD adapter conforming to version 2.0 of the SD specification faces some problems, such as worse performance of the high-frequency characteristics or severe crosstalk of signal transmissions when transforming signals between a micro SD card conforming to version 3.0 of the SD specification and the electronic device.
Therefore, providing a stable micro secure digital adapter to solve the abovementioned issues is necessary.